In our co-pending UK Application No. 9619580.5, (Publication No. 2317382), an overfill protection device (OPD) for fuel tanks is disclosed. One embodiment described in our co-pending application includes a butterfly or sleeve-type fill valve which is positioned in the fuel fill line and is controlled by a diaphragm or piston. Air in the fill line is allowed to bleed into the ullage through a valve controlled by a float during a normal filling stage. However, when the tank is almost full, e.g. about 95% full, the bleed line is shut off. This causes pressure to rise in a chamber on one side of the piston or diaphragm, resulting in movement of the piston or diaphragm which movement is transmitted to and closes the fill valve. While the constructions described in our above application function satisfactorily, the arrangement is complicated and requires a significant number of seals.